F. Ronconi

Structural characterization of Nb–TiO2 nanosized thick-films for gas sensing application

Abstract Pure and Nb-doped TiO2 thick-films were prepared by screen-printing, starting from nanosized powders. Grain growth and crystalline phase modification occurred as consequence of firing at high temperature. It has been shown that niobium addition inhibits grain coarsening and hinders anatase-to-rutile phase transition. These semiconducting films exhibited n-type behavior, while Nb acted as donor–dopant. Gas measurements demonstrated that the films are suitable for CO or NO2 sensing. Microstructural characterization by electron microscopy and differential thermal analysis (DTA) highlights the dependence of gas-sensing behavior on films properties.

Sub-ppm NO2 sensors based on nanosized thin films of titanium-tungsten oxides

Abstract We describe the preparation of thin films of TiWO 3 obtained by means of r.f. sputtering deposition followed by a thermal oxidation. The sensing characteristics of these thin films were obtained by measuring the response towards NO 2 in the interval 0.5–20 ppm and to other interfering gases like ethyl alcohol, CH 4 and CO; the influence of water vapour to the response towards NO 2 was also investigated. The material seems to be a promising detector of NO 2 for environmental monitoring.

Fragmentation of cobalt layers in Co/Cu multilayers monitored by magnetic and magnetoresistive measurements

We have monitored the structural evolution of Co(tCo)/Cu(4×tCo) multilayers when tCo ranges from 12 to 2 A. The investigation has been performed by studying their magnetization and giant magnetoresistance, since these properties are complementary in providing information about the structure of the magnetic species into the samples. In particular, in the intermediate range of thickness, we observed no correspondence between magnetic and magnetoresistive behavior. Finally, at sufficiently low thickness, the samples exhibit noninteracting superparamagnetic features. This kind of evolution has been ascribed to the progressive fragmentation of Co layers.

Role of particle size distribution on the temperature dependence of coercive field in sputtered Co/Cu granular films

The temperature dependence of coercive field and of the ratio between the remanent and saturation magnetization of granular Co/Cu films grown by sequential sputtering has been studied with magneto-optic Kerr effect measurements in the temperature range 8.5–60 K. The observed temperature dependence of coercive field does not fit any of the plausible Tx laws commonly used to describe systems of single-domain ferromagnetic particles. We worked out a generalized model, which also includes temperature effects related to nonuniform single-domain particle size distribution. The model predictions account well for the observed temperature behavior of both coercive field and ratio between the remanent and saturation magnetization.

Static magnetization direction in fcc (111) Fe/Ni multilayers

Abstract The rotation of the static magnetization was measured by Mossbauer spectroscopy as 2.5 monolayers of resonating 57 Fe were moved from inside the Fe films to the Fe/Ni interfaces in Fe/Ni (17/200 A) multilayers produced by thermal evaporation. The results indicate a remarkable dependence of the tilt angle as a function of distance from the surface.

Transport properties and magnetic disorder/order transition in FexAg100−x films

Abstract We have studied the magnetic disorder/order transition in Fe x Ag 100− x films, with x varying from 10 to 30, focusing our attention on the interplay between transport and magnetic properties. The samples have been deposited by DC magnetron co-sputtering and analyzed by magneto-optic Kerr effect and magnetoresistance measurements, with external magnetic field applied both in and out of the film plane. Magnetization and magnetoresistive results indicate that for low Fe content ( x x ⩽30) the effect of local magnetic ordering emerges and the films can no longer be considered as granular. The presence of magnetic coherence on different length scales in this regime is discussed and related to coalescence of magnetic particles and clusters formation, with increasing Fe concentration above 20%. This value appears as a critical iron content around which the magnetic disorder/order transition occurs.

GMR as a function of temperature in FeAg granular samples: the effect of magnetic interactions

We have deposited various FeAg granular systems having different iron content and we have investigated their giant magnetoresistive properties as a function of temperature. In a previous work we observed that, at room temperature, with increasing Fe content samples behaviour changes from superparamagnetic to ferromagnetic-like, and 20% is the concentration that separates these two regimes. In correspondence with this value, the dependence of giant magnetoresistance intensity as a function of iron concentration shows a maximum. With decreasing temperature, we observed that the position of this maximum shifts progressively towards lower iron concentrations. This behaviour has been discussed in terms of the presence of magnetic interparticle interactions. An analysis of the dependence of giant magnetoresistance as a function of applied field at the various temperatures is also presented.

Ferromagnetism of fcc iron films grown on nickel

Abstract Polycrystalline multilayers of Fe-Ni produced by thermal evaporation are studied; some results obtained by a vibrating sample magnetometer, Mossbauer spectrometer and X-ray diffractometer are reported and discussed. Fe seems to grown on Ni with fcc structure. Fe atoms at the Ni/Fe interface seem to have localizated magnetic moments depending on the crystallographic orientations of the grains of the substrate; the remaining Fe atoms between the two interfaces appear as highly susceptible metal.

Microstructural characterization of Fe80B20 eutectic spherulites by small‐angle neutron scattering and transmission electron microscopy

The evolution of the microstructure during the crystallization of amorphous Fe80B20 alloys has been investigated by small‐angle neutron scattering (SANS) and transmission electron microscopy (TEM). Samples with an increasing degree of crystallization were obtained by performing different isothermal heat treatments on melt spun amorphous ribbons. The TEM measurements indicate that the crystallization occurs by nucleation and growth of Fe3B spherulites which include small finely divided α‐Fe acicular crystallites. On the basis of the TEM results a model for the quantitative analysis of the SANS data has been formulated. The results show that, due to the high growth velocity of the nucleated particles, only their number increases with increasing annealing time, while their size and internal structure is not dependent on the degree of crystallization.

Mössbauer spectroscopy of amorphous Fe–Si–B alloys with different free volume content

The hyperfine field distributions of as‐cast and annealed Fe81.5Si4B14.5 amorphous alloys prepared with different quenching rates have been measured at room temperature by means of Mossbauer spectroscopy. No significant differences have been found between the spectra of as‐cast materials produced at different quenching rates, in contrast with the results of measurements of the electrical resistivity and the magnetic permeability aftereffect. This result indicates that the variation of the distribution of pressure–density fluctuations in differently quenched systems does not modify the value of the hyperfine field, which is rather dominated by the effect of the local magnetic disorder, and not substantially influenced by small variations of the quenching rate. The hyperfine field distributions of annealed ribbons are coherently shifted towards higher values with respect to the ones of as‐cast alloys. This result is related the increase of the Curie temperature, observed in these alloys after annealing, and...